This invention relates to an apparatus and a method for controlling the temperature of a substrate onto which a material is to be deposited. More specifically, the invention relates to a differential temperature control method and an apparatus for maintaining a specified temperature profile of a substrate during the formation of thin film semiconductor materials by processes which are carried out at low pressures or in vacuum. These processes are generally known in the semiconductor art and include, for example, vacuum evaporation as described in U.S. Pat. No. 3,531,335, plasma deposition as described in U.S. Pat. No. 4,064,521 or chemical vapor deposition as described in U.S. Pat. No. 2,671,739.
The properties of semiconductor materials deposited upon a substrate vary widely depending upon such factors as deposition temperature of the semiconductor material, temperature of the substrate, the rate of deposition and the like. Highly uniform semiconductor layers are required in the fabrication of photovoltaic cells which exhibit minimal batch-to-batch variations. If photovoltaic cells are to play an important role in meeting future energy needs, then large area quantities of photovoltaic devices will be needed which can be fabricated cheaply and effectively in a continuous process. The ability to uniformly control the substrate temperature and thus control a parameter which effects the final device characteristics is necessary if photovoltaic cells are to meet projected cost and efficiency goals in order to play a role in electrical power generation in the coming years.
To reduce the cost of photovoltaic cells and increase their distribution and use, researchers and manufacturers are investigating the fabrication of thin film semiconductor solar cells, such as amorphous silicon, CuInSe.sub.2 /CdS, Cu.sub.x S/CdS photovoltaic cells and the like on thin flexible substrates. The thin flexible substrates can, for example, be copper foil or high-temperature polymeric materials which have an electrically conductive metallized surface. The flexible substrates are on the order of about 25 micrometers in thickness. The thinness of the substrate presents problems in maintaining a desired temperature profile or a uniform temperature of the substrate. In addition, thin film metallic substrates such as copper, are very reflective, i.e. having low emissivity, which makes accurate determination of the substrate temperature difficult. A continuously moving substrate in an automated continuous process complicates the measurement of the actual substrate temperature; therefore, temperature control is made difficult.
The closest art of which we are aware that relates to the deposition of uniform semiconductor layers having the desired properties for photovoltaic devices is concerned with batch-type operations in which small areas or multiple small area pieces are prepared. For example, in U.S. Pat. No. 3,531,335 the temperature of a small area substrate can be controlled by affixing a thermocouple to the substrate and using the thermocouple in conjunction with a proportional temperature controller which drives heaters that provide radiant heat to the substrate. This method, and related methods which are generally used in batch or laboratory scale processes are not readily applicable to moving substrate. Furthermore, these methods do not provide the necessary spatial temperature control required for the low cost manufacture of large area, semiconductor devices such as solar cells.
Methods and apparatus for the continuous coating of metals and semiconductors on moving substrates such as metal foils and polymer films are known. However, the art known to us teaches means for merely avoiding overheating of the substrate. Thus, for example, in U.S. Pat. .No. 4,026,787, a polymer film substrate is drawn over a steel drum through which coolant is circulated during deposition of a cadmium sulfide semiconductor. The apparatus and method of U.S. Pat. No. 4,026,787 does not, however, provide control of substrate temperature sufficient to produce semiconductor layers having spatially uniform desired properties for photovoltaic devices. In particular, the aforementioned method and apparatus cannot maintain a uniform substrate temperature or desired temperature profile when the emissive properties of the substrate change, said change being inevitable as the semiconductor layer being deposited grows in thickness.
Thus, it would be highly desirable to have an apparatus which can heat and maintain the temperature of a stationary or continuously moving substrate at a constant temperature or with a desired temperature profile during the deposition of a thin film semiconductor material. It is also essential to have a method of determining the temperature of the substrate in order to control the properties of the deposited semiconductor film.